Poster: Global Change

Abs # 69: Photosynthesis during leaf ontogeny of sugarcane at elevated growth CO₂

Presenter:	Vu, Joseph C, jcvu@mail.ifas.ufl.edu
Authors	Vu, Joseph C (A)   Allen, Jr., Leon H (A)
Affiliations:	(A): USDA-ARS & Agronomy Department, University of Florida

Experiments were conducted to study the effects of rising CO₂ on leaf photosynthesis during leaf ontogeny of the C₄ sugarcane (Saccharum officinarum L.). CO₂ treatments were carried out in paired ambient-CO₂ (360 ppm) and CO₂-enriched (720 ppm) greenhouses. CO₂ exchange rates (CER) and activities of key C₄ pathway enzymes were determined on tip sections of tagged leaves at 7, 14, 32 and 48 days after leaf emergence (DAE). Leaf CER of both CO₂ treatments, measured at the growth CO₂, increased and were highest at 14 DAE. Leaf CER were the same for the CO₂ treatments at 14, 32 and 48 DAE, but were 15% higher for the CO₂-enriched plants at 7 DAE. In contrast, leaf water use efficiency (WUE) of the CO₂-enriched plants was higher for most DAE. On a leaf fresh weight basis, activities of Rubisco, PEP carboxylase (PEPC), NADP-malate dehydrogenase (MDH) and pyruvate, Pi dikinase (PPDK) generally followed the patterns of leaf CER during leaf development. Rubisco activity of CO₂-enriched plants was higher (~12%) at 14 DAE. PEPC activity of CO₂-enriched plants did not differ from that of the ambient-CO₂ plants at 7 and 14 DAE, but was lower at other DAE. MDH activity of high-CO₂ plants was up-regulated by 2.5-fold at 14 DAE, but was 10% lower than the ambient-CO₂ plants at 32 DAE. Thus, for the C₄ sugarcane, there were certain growth stage(s) of leaf ontogeny during which leaf photosynthesis could be enhanced by elevated CO₂, even though this enhancement occurred to a lesser degree compared with the C₃ species. In addition, leaf WUE for sugarcane grown at high CO₂ was greater, and this would conserve soil water and improve shoot water relations. These factors could contribute to an enhancement in biomass accumulation of sugarcane under elevated CO₂.